A great deal of digital circuits includes an oscillator for generating a clock. Recently, as an amount of data handled by digital circuits grows, an oscillation frequency of an oscillator has been on the increase. For example, since a data rate of a transmitter that transmits serially transmitted image data or a receiver that receives serially transmitted image data reaches even up to 5 Gbps, a frequency of a few GHz is required as a clock for sampling.
An LC oscillator or a ring oscillator has been used as an oscillator. The LC oscillator easily generates a clock of a few GHz. However, the integration of an inductor on a semiconductor substrate requires a high frequency process, increasing costs. Further, since inductance is fixed, it is difficult to vary an oscillation frequency.
Meanwhile, the ring oscillator is configured by arranging a delay circuit as a base in a ring shape and is configured through a CMOS process or BiCMOS process, which is thus advantageous in terms of costs. In addition, by changing the bias condition of the delay circuit, a delay time thereof can be changed to vary an oscillation frequency.
The ring oscillator is configured by using a plurality of delay circuits connected in multiple stages, and a clock frequency is increased as a delay amount of each of the delay circuits is reduced. However, there is a restriction in a delay amount of the delay circuits that can be configured through a general semiconductor manufacturing process and it is difficult to generate a clock of a few GHz.